1. Field of the Invention
This invention is related to measuring instruments, and in particular to test probes and a system and a method of attaching the test probes to systems under test.
2. Related Art
The cost of testing a printed circuit board (“PCB”) assembly is a significant part of the overall cost of producing a product that contains one or more PCBs. As PCB assemblies become more sophisticated and complex and there is greater time-to-market pressure, there is increasing interest in improving methods used to test PCB assemblies. As the individual components become smaller and more are added to the PCB assemblies, the test probes of the measuring instrument used to test these devices have to be made smaller and more efficient.
In general, probing refers to the act of connecting a measuring instrument to a signal access point on a system-under-test (“SUT”). Probing is required to observe signals and the shape of a waveform as it exists at a particular location, i.e., at a particular access point. A typical example may be connecting an oscilloscope (the measuring instrument) to the pin of a dynamic random-access memory (“DRAM”) device, which may take the form of a TSOP (Thin Small Outline Package). The probe is a device that establishes a high-fidelity electrical contact between the measuring instrument and a signal access point and “siphons” off a portion of the signal energy and transfers it to the measuring instrument. During this acquisition and transfer, the signal is exposed to the environment and is susceptible to corruption.
Probes typically have a signal pin and a ground pin. The signal pin is attached to a signal access point on the SUT and the ground pin is attached to a ground access point on the SUT. The act of probing may significantly change the waveform being observed, resulting in measurement error. This may happen because of the probe itself as well as the method and the apparatus used to attach the probe to the SUT. As was noted earlier, the signal is exposed to the environment during its “pick-up” and transfer to the probe and during its transmission to the measuring instrument and may be corrupted during any of these stages. Thus the manner of attaching the probe to the access point is of great importance and must be accomplished in such a way that it does not degrade, modify, or alter the signal being measured.
In high performance designs (which are typically classified as interfaces of signal lines having fast signal rise times and high frequencies), accurate probing is the key to accurate measurements. In such high performance designs, transmission line effects must be taken into account when designing the physical layout of the system. Accordingly, a probe, when it is connected to an access point of the system, may itself be considered a parasitic component that affects signal integrity. Further, the method of connecting the probe to the SUT plays a vital role in determining if the signal that is “picked-up” from the SUT gets accurately transferred to the probe. Thus, with today's high performance designs, there is an even greater need for improved probes as well as techniques that preserve the integrity of the signal during its “pick-up” and subsequent transfer to the probe.
FIG. 1 shows a schematic block diagram illustrating an SUT 100 being tested by a conventional probe 104. In FIG. 1, the SUT 100 may be a printed circuit board (“PCB”) that may have multiple surface-mounted and chip scale packages attached. In FIG. 1, a thin small outline package (“TSOP”) 102 is shown, which may be, as an example, a DRAM device. TSOPs are surface-mount devices with a very fine pitch between pins and are installed on the surface of the SUT 100.
Probe 104 has a signal pin 106 and a ground pin 108 and is used to connect a signal access pin on the SUT 100, to a measuring instrument (not shown). To measure a signal, the signal pin 106 of probe 104 is connected to a signal access pin of TSOP 102, which may be one of the pins 120 of TSOP 102. To complete the connection, the ground pin 108 must be attached to a ground access point 112 on the SUT 100.
In general, a PCB may have a flat plate or base of insulating material containing a pattern of conducting material, and may be single-sided, double-sided, or multi-layer, with several power and ground planes. A ground plane of the PCB may be reached through a ground via, which is a plated-through hole in the PCB used to route a trace vertically in the PCB, that is, from one layer to another. If a ground via is not located on the PCB near the ground pin 108 of the probe, then a grounding wire 110 is required to connect the ground pin 108 of the probe 104 and the ground access point 112 of the PCB.
In FIG. 1, grounding wire 110 connects ground pin 108 of the probe 104 to ground access point 112 of the PCB 100. This connection forms loop area 114 that is a source of signal integrity problems. Because of the larger loop area created by the length of the grounding wire 112, it acts as an antenna to pick up any stray noise in the SUT 100. Additionally, such a connection is highly inductive in nature, and degrades the rise time of the signal that is actually transferred to the probe. This leads to inaccurate measurements, particularly in the case of high performance systems. The loop area 114 is used to illustrate the extent of such problems, with a larger area being more conducive to noise problems and greater inaccuracies in measurement.
Additionally, the use of a ground via to make a connection to a ground plane presents other potential problems. Generally, use of a ground via results in a relatively small loop area, thus allowing for accurate testing results. However, repositioning the probe requires great care and repeated probing will often cause damage to the ground via. Moreover, even the slightest movement may knock the ground pin from its desired location. And effective use of a ground via requires that it be located near each signal access point that is to be tested.
Another approach is to use a grounding wire soldered to the PCB and the probe. An advantage of this approach is that it provides a solid connection, with little possibility of damage. However, because a large loop area is formed, this approach may be acceptable for slow speed interfaces, but will not have the accuracy required for measuring high speed signals.
Therefore, there is a need for a probe capable of accurately measuring high speed signals and that is also able to be easily repositioned for additional testing without causing damage to the SUT or to the probe.